The 11/11 Dupe, also known as the Jump Dupe, was a massive duplication glitch on the server that occurred on November 11, 2016. The easy way to duplicate items was leaked to the 2b2t community by RusherIsAutistic, causing mass duping across the entire server.
The dupe involved tossing an item out and instantly disconnecting the server. When logging back in, the item would be on the ground and in the player's inventory, which sparked the mass dupe. This was the largest duplication glitch that the server had ever seen before then, since an entire inventory could be duped at once.
On November 19, a client by kinorana, kinodupe, was made to easily perform the 11/11 Dupe, and was spread among the community. Players were able to drop their entire inventory on the ground at once and automatically disconnect, making it possible to dupe an entire inventory at once. The client was backdoored, however, and people who duped using the client had their stashes leaked on the 2b2t Reddit.
However, a specific group of players, the 4th Reich, had another way to duplicate. This was the first method used before other clients were made consisting of other types of kicks that could be used. This method involved using two Minecraft windows with which you would login to one and drop an item before being kicked on the other.
The 11/11 Dupe caused the amount of items in the server to overflow, with most players having double chests of God Apples and other rare items. This caused the 2b2t economy to crash, with items from the dupe still in mass circulation today.